iMBEDDmG IN PARAFFIN 203 



191. Dehydration and clearing. The material to be cut is passed carefully 

 through the grades of alcohol to 95% (Sec. 178). It should remain in 95% alco- 

 hol for at least several hours or more, and is then placed in absolute alcohol, 

 generally in a vial, and this should be poured off and rtoewed after an hour 

 or two. The material should be left in absolute alcohol from four to eight 

 hours or over night, unless the object be very small. It ought then to be free 

 from water (dehydrated) and ready for the clearmg agent, which will remove 

 the absolute alcohol and also dissolve the paraffin, so that the latter may re- 

 place the former throughout the tissue. The clearing agents most frequently 

 used are chloroform and xylol. Chloroform acts more rapidly, but there is 

 less danger of shrinkage with xylol. However, the chief danger of shrinkage 

 lies in imperfect dehydration. 



Three mixtures of the clearing agent (chloroform or xylol) With absolute 

 alcohol are necessary to insure the gradual replacement of the latter by the 

 former. These are (1) one fourth clearing agent, three fourths absolute 

 alcohol : (2) half and half clearing agent and absolute alcohol ; (3) three 

 fourths clearing agent and one fourth absolute alcohol. The material is 

 passed through these mixtures in the above order and then into the pure 

 clearing fluid, either chloroform or xylol. When chloroform is used the 

 material need not be left more than from four to eight hours in each mixture, 

 and less if the object be small. If xylol is used, the material should be 

 left at least twelve hours in each mixture, and a longer time will do no harm. 

 It should not remain in pure chloroform more than twelve hours before 

 paraffin is added, and a shorter time is generally better; but it may be 

 left in pure xylol a longer time, and even a day or more with advantage. 

 Besides removing the absolute alcohol the clearing agent renders the tissues 

 more transparent, that is, "clears" them. 



193. Infiltration. Small pieces of paraffin are now added to the chloroform 

 or xylol to the point of saturation and beyond. At this time the vials may 

 be placed on the top of the oven, where they will be warmed, thus allowing 

 more paraffin to dissolve. 



The best form of paraffin bath is a square or rectangular hot-water oven, 

 with a door at the side and one or more shelves within. This should be 

 heated by gas or by an electric coil with a thermostat arrangement to keep 

 the oven at a constant temperature of about 52° C. The temperature may 

 run as high as 56°, or probably higher if the dehydration has been perfect, 

 but in general the temperature should be kept low. 



Material in chloroform and paraffin is placed in the bath and the vial un- 

 corked. The chloroform will be driven off after a number of hours, leaving 

 the material in pure melted paraffin. This process should not be hastened ; a 

 day or two in the paraffin bath will generally give the most satisfactory results. 

 Tasting the paraffin is the best test of the removal of the chloroform ; should 



